A number of different objects display different images, or different views of an image, depending on the angle by which the object is viewed. Some objects, such as holographs, utilize diffraction to separate white light into in its spectral components. A diffraction grating, having 20,000 to 50,000 lines per inch, reflects or transmits different portions of the incident spectrum. The portions are seen as a view of an image which changes as the angle of incidence changes. The process of manufacturing the diffraction grating requires great accuracy and is very expensive.
Rather than utilize the diffraction principal, some objects are provided with embossed foil having far fewer lines per inch which reflects white light as light and dark lines. The reflected lines appear to shift as the viewing angle changes, but changes in color are not produced.
Other embossed objects are printed on with different colors. The arrangement of the printed pigments in relation to embossed lines can establish a moire pattern which interferes with the intended image. For some prints the objectionable moire patterns must be overcome by printing different colors as dots at a different periodicity or angle than the periodicity or angle of the embossed pattern.
Yet other objects utilize a lenticular construction in combination with color pigments. Lenticular films have a number of tiny, semi-cylindrical lenses, known as lenticules, which are typically formed as parallel ridges embossed on the base side of the film. The lenticules extend in parallel across the entire surface of the film and alter the manner in which the underlying emulsion is exposed by the subject, and any intervening, color filters.
Lenticular films are often used to generate a stereoscopic effect by revealing left and right images as the viewing angle changes. A lenticular print can similarly be made through printing techniques using half-tone dots. However, individual regions of the image are not oriented so that the colors change according to the viewing angle.
Yet other objects, such as those disclosed in U.S. Pat. Nos. 4,932,685 and 4,968,064, utilize embossed film in which periodic variation in an optical characteristic of the medium extends in a given direction for a particular region of a given image. Each region is distinguished from any adjacent region in that the optical characteristic is oriented differently. However, this process can be very expensive in that it requires each image to be individually engraved. Further, it requires registration of the color image printed upon the embossed medium with the locally engraved regions.